Preheat the oven to 140C/120C fan/gas 1. Select a large heavy casserole pot (with a lid) that will hold the lamb snugly. Separate the garlic into cloves and peel them.

Heat the olive oil in the casserole over a medium-high heat, season the lamb and sear it in the pot to colour it all over. Tip out any excess fat and place the lamb fat-side down. (If the butcher has given you the knuckle you can tuck this alongside). Add the garlic, bay leaves and apple juice, which should come to the boil almost instantly. Put the lid on the pot and cook in the oven for 5 hours, turning the meat over halfway through (ie, after 2½ hours). Cut the rhubarb into lengths and add to the pot 30 minutes before the end of cooking.

Transfer the lamb to a warm serving dish. Skim any excess fat off the roasting juices, discard the bay leaves (and the knuckle if present). Whiz the contents of the pot in a blender until smooth. Return to the pot and season with salt, pepper and caster sugar. Bring to the boil, skim off any surface foam and simmer vigorously to reduce by about a third. Transfer to a serving jug.

Carve the lamb into hearty chunks – it should be meltingly tender – serve with the sauce and, if wished, some buttered spinach.